Filterting for categories, omit empty lists in frontmatter

src/writer.js

@@ -56,14 +56,26 @@ async function writeMarkdownFilesPromise(posts, config ) {
 
 async function loadMarkdownFilePromise(post) {
 	let output = '---\n';
-	Object.entries(post.frontmatter).forEach(pair => {
-		const key = pair[0];
-		const value = Array.isArray(pair[1])
-			? (pair[1].length === 0 ? "" : "\n  - \"" + pair[1].join("\"\n  - \"") + "\"")
-			: '"' + (pair[1] || '').replace(/"/g, '\\"') +'"';
-		output += key + ': ' + value + '\n';
+
+	Object.entries(post.frontmatter).forEach(([key, value]) => {
+		let outputValue;
+		if (Array.isArray(value)) {
+			if (value.length > 0) {
+				// array of one or more strings
+				outputValue = value.reduce((list, item) => `${list}\n  - "${item}"`, '');
+			}
+		} else {
+			// single string value
+			const escapedValue = (value || '').replace(/"/g, '\\"');
+			outputValue = `"${escapedValue}"`;
+		}
+
+		if (outputValue !== undefined) {
+			output += `${key}: ${outputValue}\n`;
+		}
 	});
-	output += '---\n\n' + post.content + '\n';
+
+	output += `---\n\n${post.content}\n`;
 	return output;
 }